\documentclass[11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{booktabs}
\usepackage{array}
\usepackage{siunitx}
\usepackage{tabu}
\usepackage{longtable}
\usepackage{makecell}
\usepackage{multirow}
\begin{document}
\begin{table}
	\centering
	\begin{tabu} to 1.1\textwidth{| X[l] | X[l] | X[l] | X[l] | X[c] | X[c] |}
		\cline{2-6}
		\multicolumn{1}{c|}{}         & \multicolumn{3}{c|}{Voltage measured (V)} & \multirow{2}{*}{\thead{Mean               \\ voltage (V)}} & \multirow{2}{*}{\thead{Standard\\ deviation}}\\
		\cline{1-4}
		Temperature (\SI{}{\degree}C) & Replicate 1 & Replicate 2 & Replicate 3   &                                 &         \\
		\hline
		10                            & 1.79        & 1.78        & 1.79          & $\frac{1.79+1.78+1.79}{3}=1.78$ & 0.00577 \\
		\hline
	\end{tabu}
\end{table}
\end{document}
